A sterile artificial substitute for the bearing surface of only one femoral condyle and corresponding tibial condyle of the knee (medial or lateral condyle) implanted during primary or revision knee replacement. It typically consists of femoral and tibial components and a knee insert which are made of metal or polyethylene (PE); fixation devices for implantation (e.g., screws) may be included and implantation may be performed with or without bone cement. The device is indicated in unicompartmental disease and/or damage (e.g., from arthritis) to the bearing surfaces of the knee.